Electrophotography employing a film having a thin charge retentive coating on a conductive web



March 24', 1970 BRODIE 3,50 ELECTROPHOTQGRAPHY EMPLOYING A FILM HAVING A THIN CHARGE RETENTIVE COATING ON. A CONDUCTIVE WEB Filed Dec. 5, 1966 F|G.| I 2 I? 4 71 x 1 |4 x //GLASS PLATE y TRANSPARENT ELECTRODE a .aw azaa PHOTOCONDUCTOR 8 'IIIIIIIIlW/III/IIIIIA INSULATIVE FILM CONDUCTIVE I59 5 H CONDU-CTIVE ELECTRODE A5 l2 W M I FIG.2 %(3 0.0; 2 SOURCE SOURCE 28 I4 /26 21 FIG. 3

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won BRODIE BY o 200 400 600 a VOLTAGE ATTORNEY United States Patent U.S. Cl. 355-16 5 Claims ABSTRACT OF THE DISCLOSURE may be made extremely thin for greater charge retention and improved contrast while facilitating handling of the web for development and viewing. Such an improved electrophotographic web is especially useful for, but not limited in use to, microfilm printers and oscilloscope cameras of the electrophotographic type, whereby contrasting prints are obtained on an inexpensive and durable web material.

DESCRIPTION OF THE PRIOR ART Heretofore, xerographic images to be developed have been formed on thin sheets of dielectric material such as polystyrene or formed on polystyrene film as coated on a metal or conductive glass plate. Such arrangements are described in U.S. Patent 2,825,814 issued Mar. 4, 1958. The problem with these prior art devices is that, as regards the thin sheets of dielectric material, the thinner the sheet the more chrage it can store, thus, in order to store a sufficient amount of charge the sheet must be very thin, as of, for example, on the order of a few microns thick. Such a thin sheet of dielectric is difiicult to handle for image development and for subsequent viewing. Moreover, such films do not lend themselves to powder development as the heating of the film to fix a powder developer typically deforms or destroys the film.

When such thin films of dielectric are coated onto conductive glass or metal backing plates the film is much more durable and more easily handled. However, such composite plates are relatively bulky and expensive.

Moreover, such plates do not lend themselves to development by liquid development channels as they are not readily drawn against the inking slot for sealing thereof.

In the electrostatic printing art, pin electrodes and character printing wheels have been employed for laying down line type charge image patterns on electrographic paper. The paper comprises an insulative charge retentive layer coated on a conductive paper backing member. The charge retentive layer isabout microns thick and the paper backing is several times the thickness of the coating. Such paper is described in U.S. Patent 3,182,333 issued May 4, 1965. This type of paper is relatively inexpensive, durable, easily handled and readily developed either by liquid or dry powder development methods. However, as far as is known this type of paper has not been employed as a film for receiving electrophotographic images.

In the present invention, it has been discovered that such electrographic paper is well suited for use in electrophotography especially when the thickness of the charge retentive layer is substantially less than 10 microns, with 4 microns being a particularly suitable thickness.

3,502,408 Patented Mar. 24, 1970 This paper provides sufiicient contrast, permits large area image retention and development, is easily handled, readily developed by liquid or dry powder toners, and is only slightly more expensive than ordinary paper.

The principal object of the present invention is the provision of an improved means for printing electrophotographic images.

One feature of the present invention is the provision of an electrographic web for receiving, retaining and printing electrophotographic charge images, such web having a charge retentive coating on a conductive web portion.

. Another feature of the present invention is the same as the preceding feature wherein the conductive web portion is a conductive paper.

Another feature of the present invention is the same as any one or more of the preceding features including means for urging the web into contact with a photoconductive member for depositing the electrophotographic image upon the charge retentive coating of the web.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

FIG. 1 is a schematic diagram depicting an electrophotographic apparatus employing features of the present invention.

FIG. 2 is a schematic diagram of an electroradiographic apparatus employing features of the present invention,

FIG. 3 is a schematic cross section of an alternative electrographic paper for use in the apparatus of the present invention,

FIG. 4 is a plot of charge transfer to charge retentive coatings of different thickness on conductive paper, and

FIG. 5 is a schematic perspective view of a liquid toner developer for developing strips of electrographic web.

Referring now to FIG. 1 there is shown an electrophotographic camera 1 employing features of the present invention. The camera 1 is not shown in detail and includes a dark box, not shown, which contains a lens to focus the image of an object 3 onto a focal plane at the rear of the dark box. Suitable electrophotographic cameras are shown in U.S. Patents 2,297,691 and 2,277,013. A film holder, not shown, is disposed at the focal plane of the camera and includes a sandwich 4 of a number of elements for producing and recording an electrostatic image of the object 3.

The sandwich 4 of elements includes an optically transparent plate 5 as of glass 0.250" thick which is fixedly held in position within the film holder. An optically transparent electrically conductive electrode 6 is formed on the glass substrate 5, as by vapor deposition. A suitable material for the conductive electrode 6 is a very thin metal film deposited to a thickness of about one hundred angstroms. A photoconductor layer 7 is formed on the transparent electrode. A suitable photoconductor material is selenium deposited as by vapor deposition to a thickness of 0.0005 to 0.002".

A replaceable electrographic web 8 for receiving and retaining a charge image of the object 3 being photographed is positioned between the photoconductor layer 7 and a conductive electrode backing plate 9. The plate 9 is preferably urged toward the glass plate 5, as by springs 11, to hold the electrographic web 8 in uniform nominal physical contact with both the photoconductor layer 7 and the backing electrode plate 9. It should be understood that under these conditions a thin layer of air a few microns thick separates the adjacent web 8 and photoconductor 7' except at a few points due to the microscopic surface roughness.

A DC. voltage source 12 as of 400 to 1000 v. is connected across the transparent electrode 6 and the conductive backing plate '9 via a reversing switch 13 and a timing switch 14. The shutter 2 may also be synchronized with the timing switch 14.

The electrographic web 8 comprises a conductive web member 15 as of conductive paper or other conductive flexible material. A charge retentive coating 16 of a dielectric insulative material, such as polyethylene, is formed on the conductive web 15. A suitable electrographic web 8 is lot 6550-B electrographic paper marketed by Plastic Coating Corporation of Holyoke, Mass.

In operation, the photoconductor 7 is first flooded with light from a suitable source such as an incandescent lamp 17 to discharge to ground any residual charge images on or in the photoconductor layer 7. The lamp 17 is then extinguished and switch 14 closed for suitable exposure times such as, for example, to of a second depending upon the photoconductor material and the intensity of the photo image falling upon the photoconductor layer 7. At the end of the exposure, the switch 14 is opened and the shutter 2 closed. During the exposure time, the photoconductor 7 becomes conductive in a conductive pattern conforming to the optical image of the object 3 falling upon the photoconductor 7. The applied voltage from the source 12 is set to a sufficient amplitude such that, in the presence of the optical radiation, the voltage applied between the inner face of conductive portions of the photoconductor 7 and the conductive web 15 exceeds a certain threshold voltage such as, for example, 385 volts. Above this threshold voltage, a charge image of the object 3 is transferred to the charge retentive coating 16 of the electrographic web 8. In a typical example, employing a selenium photoconductor 7 and model 65-50-B electrographic paper 8, the voltage of source 12 is 500 volts.

Once the charge image has been transferred to the electrographic web 8, the web is removed from the camera 1 and developed by conventional dry powder or liquid toner methods. A particularly convenient liquid toner development apparatus for developing strips of electrographic web 8 is shown in FIG. 5. More particularly, the web 8 with the charge image 21 deposited thereon is passed under a hollow inking channel 22 through which is flowing a liquid toner at less than atmospheric pressure. An inking slot 23 is cut through the lower wall of the channel 22. Atmospheric pressure pushes the web 8 up against the slot 23 sealing same and bringing the charge image into contact with the liquid toner. The pigment particles of the toner have a charge which is opposite to the charge image to lie developed and, thus, are drawn to and held to the charge image 21 thereby developing same at 24. The flexibility of the web 8 facilitates liquid development of the images since flexure of the web 8 permits sealing of the inking slot 23 as the web 8 is drawn past the slot 23.

Referring now to FIG. 2, there is shown an alternative embodiment of the present invention for taking X-ray electrographs. More specifically, the apparatus is essentially the same as that of FIG. 1 except that the dark box is eliminated and the photoconductor is replaced by a material such as selenium which is normally insulative but which is rendered conductive upon radiation by X-rays. This X-ray sensitive material is deposited as a layer 26 upon an X-ray transparent electrically conductive plate 27 as of aluminum. An object 28 to be X-rayed is placed above the plate 27. A source of X-rays 29 is disposed above the object 28 to direct its X-rays through the object 28 onto the X-ray sensitive layer 26. An X-ray picture is taken by simultaneously exposing the object to X-rays and applying the voltage to the electrodes 9 and 27 from the source 12. The electrostatic image of the object 28 is formed on the electrographic web 8 in the same manner as previously described with regard to FIG. 1. The web 8 is developed as aforedescribed.

Referring now to FIG. 3, there is shown an alternative electrographic web 8'. In this case, the charge retentive dielectric insulative layer 16 is coated on a conductive web portion 15' which includes a conductive layer 31, as of silver, deposited in vacuum on a flexible web 32 which web 32 may be non-conductive or conductive. Electrical connection to the web 8' is made by making connection to the conductive layer 31. Electrographic web 8' is used in the same manner as previously described for the electrographic web 8 except that electrical connection to the source 12 is made through the conductive layer 31 instead of through the layer 32.

In a preferred embodiment of the electrographic webs 8 and 8', the dielectric charge retentive layer 16 is preferably less than 10 microns thick a particularly suitable web has a layer 16 which is 4 microns thick. The higher the dielectric constant of the coating 16 and the thinner it is the more charge it can retain for a given voltage and thus the greater the contrast of its images.

Referring now to FIG. -4 there is shown a plot of charge transfer in mano-coulombs per cm. versus applied voltage for three different plastic coating layers 16 of different thickness. Paper B is the preferred paper of the three depicted since its threshold voltage is a clearly defined point and it has a steep slope of 850 F. capacitance. The greater the capacitance and the more abrupt the threshold point, the more contrasting will be the electrostatic images formed on the charge retentive layer 16.

As regards the conductive web 15, the less its resistance the less time it takes to transfer the charge image to the charge retentive layer 16. The charge transfer time is approximately equal to 10 times the dielectric relaxation time (the product of 8.85 x l0 Kp, where K is the dielectric constant of the layer 16 and p is the resistivity of the conductive web 15 in ohm cm.). Using a high dielectric constant layer 16, the charge transfer time is about 1 second for a resistivity of the web 15 of 10 9 cm. Thus, relative resistive webs 15 may be employed but resistivities less than 10 0 cm. are preferred. As used herein conductive web is defined to mean a web having a resistivity of 10 9 cm. or less. Also as used herein, electrophotography and electrophotographic apparatus is defined to mean apparatus for producing electrical charge image patterns on an electrographic web such patterns conforming to a pattern of radiation which is either reflected from the object or absorbed by the object. Such radiation may be in the visible range or in the invisible range, such as that produced by X-rays.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention can be made without departing from the scope thereof it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. Electrophotographic apparatus including; means forming an electrophotographic member having a charge retentive surface for receiving an electrophotographic charge image to be developed thereon; means forming a normally insulative member disposed adjacent said charge retentive surface of said electrophotographic member, said normally insulative member being made of a material which is rendered electrically conductive by application of a pattern of radiation to said member, means for applying an electrical potential across said normally insulative member and said charge retentive surface of said electrophotographic member, means for producing an image pattern of radiationon said normally insulative member to cause said member to become conductive in a pattern corresponding to the applied image pattern of penetrating radiation, said applied potential being of suflicient amplitude to cause an electrical charge image to be formed on said insulative charge retentive surface with a pattern conforming to the applied image pattern of the radiation; the improvement wherein, said electrophotographic member comprises a conductive flexible paper web, a coating of insulative dielectric film substantially less than 10 microns thick formed on said conductive web, said insulative dielectric film forming said charge retentive surface for receiving and retaining the electric charge image to be developed thereon.

2. The apparatus according to claim 1 wherein said conductive paper web is impregnated with an electrically conductive substance.

3. The apparatus of claim 1 wherein said normally insulative member which is conductively sensitive to radiation is a photoconductor sensitive to optical radiation to render said member conductive.

4. The apparatus of claim 1 wherein said normally insulative member which is conductively sensitive to radiation is a material sensitive to X-ray radiation to render said member conductive.

References Cited UNITED STATES PATENTS 2,825,814 3/1958 Walkup 25049.5 3,182,333 5/1965 Amada et a1. 346-74 3,298,831 1/1967 Lau et a1 96-1.5 3,316,088 4/1967 Schaifert 961.5

JOHN M. HORAN, Primary Examiner Disclaimer 3,5O2,408.I4)or B'roclz'e, Palo Alto, Calif. ELECTROPHOTOGRAPHY EMPLOYING A FILM HAVING A THIN CHARGE RETEN- TIVE COATING ON A CONDUCTIVE WEB. Patent dated Mar. 24, 1970. Disclaimer filed Feb. 11, 1974, by the assignee, Varian Associates. Hereby enters this disclaimer to all claims of said patent.

[Oflicial Gazette August 27, 1.974.] 

